Apparently some users are having performance issues on the PC version, even with cards that should be more than capable of maxing out the game, and that reducing graphics settings and even resolution doesn't fix the issue. However, there have also been reports that turning TXAA off and using FXAA instead fixes the problems. Just turning down TXAA doesn't fix it, you have to actually disable it. Seems like there's a problem with the way TXAA is implemented so it's a much bigger performance hog than it should be.